1. Field of the Invention
The present invention relates to a method for granulating a cationic surfactant and, more specifically, relates to a method for granulating a powdered di (long-chain alkyl) quaternary ammonium salt ("dialkyl quaternary salt" hereinbelow) to form granules having good storage stability and good water dispersibility.
2. Description of the Prior Art
Dialkyl quaternary salts are generally produced in a solution state. However, there are many fields of application where powdered or granular dialkyl quaternary salts are convenient. For example, in the case of a softening agent, when only a very small amount of a powdered dialkyl quaternary salt is added to a wash rinsing solution, a fabric-softening effect comparable to that of a conventional commercially available liquid softening agent can be obtained. However, a powdered dialkyl quaternary salt is not substantially manufactured due to its dusting effects and handling inconvenience. Accordingly, attempts have been made to produce granules of dialkyl quaternary salts from powdered dialkyl quaternary salts. However, the granules obtained from the use of conventional nonionic surfactants have poor water dispersibility after storage. This is based on the facts that the transition point is rapidly decreased by absorbing a large amount of moisture under a high humdity condition due to a high humidity sensitivity or hygroscopicity of the dialkyl quaternary salts, while the dialkyl quaternary salts rapidly desorb the moisture when humidity is decreased. That is, dialkyl quaternary salts absorb moisture to swell under an elevated temperature and high humidity and the molten dialkyl quaternary salt powder particles aggregate together with the decrease of the transition point and, then, are solidified upon dewatering.
It is known in the art that hygroscopic substances are generally mixed with inorganic powder capable of absorbing water as water of crystallization such as anhydrous sodium sulfate and anhydrous magnesium sulfate to granulate the hygroscopic substances. However, the amount of water which can be retained as water of crystallization in the inorganic substances is limited and, further, once absorbed as water of crystallization, the water is not desorbed again at a room temperature. Therefore, the water-proofing effect of these inorganic substances is lost during storage and the aggregation of the dialkyl quaternary salt powder particles cannot be prevented.
Furthermore, when the granulated particles are stored under an elevated temperature and high humidity, water particles dew condensed on the surfaces of the granulated particles cause the aggregation on adhesion of the granulated particles. Water-soluble inorganic substances are partially dissolved in dew condensed water and accelerate the aggregation or solidification of the granulated particles bonding the granulated particles together when the inorganic substances recrystallizing upon vaporization of the dew condensed water.
Alternatively, it is proposed to mix powdered dialkyl quaternary salts with only slightly water-soluble inorganic substances. However, calcium carbonate, aluminum hydroxide, and similar only slightly water-soluble inorganic substances are susceptible to permeation of water due to strong affinity thereof to water. Bentonite and similar only slightly water-soluble inorganic substances are tend to absorb water to swell and enlarge the distances between the adjacent inorganic substance particles, which cause easy permeation of water through the particles. Consequently, water dispersibility after storage is poor the prevention of caking is difficult.